Al—Si alloys exhibit a eutectic reaction with the liquidus/solidus temperatures equal at about 11% Si by weight, with some modification when other alloying elements are present. Hypoeutectic alloys, i.e. those with lower silicon content than eutectic, generally exhibit primary aluminium particles in a two-phase eutectic, as do eutectic alloys. Hypereutectic alloys, i.e. those with higher silicon content, exhibit primary Si particles in a two-phase eutectic. The presence of primary Si particles is beneficial for resistance to galling (adhesive wear) and for hardness. In contrast to the situation in cast products, in which these particles can be gross and angular, adversely affecting mechanical properties and machinability, atomized Al—Si hypereutectic powder contains primary Si particles which are refined and cuboid with a 5 to 10 micrometre average particle size, which are considered to be optimal for wear resistance and machinability. Further, precipitates arising from the presence of impurities, such as the β-AlFeSi phase, which can arise in cast products, are not significant in atomized powders.
Further background is given in WO94/29489, which discloses and discusses the use of mixtures of hypereutectic and near-eutectic aluminium-silicon powders to obtain pressed and sintered powdered metal, and in WO02/27047, which discloses the use of zinc or zinc-based powder as a sintering aid for such mixtures. As evidenced by FIG. 3 of the latter publication, the as-atomized powder mixtures are not easily compressible, a situation which is markedly improved by annealing the powders. However, and significantly in industrial applications, an annealing step will incur additional manufacturing cost.